DNA damage and DNA repair have many important consequences for human health. Cancer and developmental defects have been associated with congenital deficiencies in DNA repair. Cancer treatment is often based upon damaging DNA or inhibiting DNA repair in the diseased tissue. In part because of its implications for human health, nucleotide excision repair (NER) has been the subject of intense investigation and a major focus of our research for nearly four decades. Based largely upon pioneering work in our laboratory, a close relationship between DNA repair and transcription has been documented in mammalian, yeast, and bacterial cells. RNA polymerase (RNAP) is a prime candidate for an essential role in this relationship, but we still do not understand exactly how it participates. Results of numerous investigations indicate that RNAP interacts with other proteins involved in DNA repair, but current ideas about the details of the interactions are sometimes contradictory. This is particularly true for transcription coupled repair in human cells in which both NER and base excision repair have been implicated. Although our ultimate goal is to understand the mechanism of transcription-coupled NER (TC-NER) in human cells, on the basis of past experience we believe that important general principles may be revealed by studying the process in the simplest systems in which it can be demonstrated. Therefore, we will focus upon the monomeric RNAP of bacteriophage T7 and the multisubunit RNAP of Escherichia coli. I. Having obtained evidence that transcription of a gene by the T7 RNAP results in enhanced repair of the transcribed strand relative to the non-transcribed strand (the hallmark of TC-NER) after UV-irradiation, we will study the biochemical basis of this effect, including the requirements for other proteins such as Mfd and mismatch proteins. II. We will identify properties of the E. coli RNAP subunits involved in TC-NER by testing well characterized mutants (rpoA, rpoB, rpoC, rpoD) for UV sensitivity. UV sensitive mutants will then be analyzed for global genomic NER and TC-NER. III. We will measure DNA turnover in the undamaged lac operon when it is expressed, or repressed, comparing the frequency of "gratuitous" repair synthesis in each strand using an approach developed in this laboratory. In addition, the nature of repair synthesis following thymine deprivation, its dependence upon transcription, and the possibility that it may reflect "gratuitous" TC-NER will be assessed. Gene expression profiles during thymine deprivation will be assessed by microarrays. The results of these experiments are relevant to an understanding of the adverse consequences of folate deprivation in humans.